The present invention relates to a blind threaded nut, which has improved resistance to shear and tensile forces and improved resistance to applied torque.
Conventional blind threaded nuts, i.e. nuts that are secured by insertion from one side of the workpiece, have a number of disadvantages when considered for applications where the nut is likely to be subjected to high load and high torque, in spite of having the advantages of easy and low-cost application and a simpler workpiece structure.
In such applications, a weld nut, i.e. a nut which is welded to the blind side of the workpiece and to which a secondary component is bolted, is generally used. The use of weld nuts has the disadvantage that they require more expensive application equipment than conventional blind nuts and, because access is required to the blind side during mounting of the weld nut, this may result in undesirable weakening of the workpiece structure on which the nut is being mounted, especially where the weld nut is being applied to a hollow structural member such as a box section.
Where a conventional blind threaded nut is used in an application where the nut is likely to be subjected to high load and high torque, a number of problems can develop.
Firstly, if the hole in which the nut is mounted has been poorly prepared and has an uneven surface due to burrs etc., then as the setting load is applied to the blind nut, a collapse bulge will start to be formed as normal but as the maximum load is applied, the fixing will tilt as the bulge adapts to the uneven workpiece surface. Secondly, once the bolt is in place and in particular when the joint is being serviced or re-used, an operator may apply excessive torque which will tend further to flatten the collapse bulge and possibly promote cracking of the resultant flattened collapse bulge.
Further, if an excessive shear loading is applied to the joint, then the fixing will initially buckle sideways, the wall of the fixing will shear and the bolt will incline as the shear loading is transferred to the bolt alone. The loading is then translated into combined shear and bending, resulting in premature joint failure.
Further, if an inexperienced operator does not sufficiently torque the bolt, then any sideways thrust is taken on the thread of the fixing, causing a tendency for the collapse bulge to straighten. If a sudden and reverse loading is applied, then the joint will suffer premature failure.